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Monotonic transformations are widely employed in statistics and data analysis. In computer experiments they are often used to gain accuracy in the estimation of global sensitivity statistics. However, one faces the question of interpreting results that are obtained on the transformed data back on the original data. The situation is even more complex in computer experiments, because transformations alter the model input-output mapping and distort the estimators. This work demonstrates that the problem can be solved by utilizing statistics which are monotonic transformation invariant. To do so, we offer an investigation into the families of metrics either based on densities or on cumulative distribution functions that are monotonic transformation invariant and we introduce a new generalized family of metrics. Numerical experiments show that transformations allow numerical convergence in the estimates of global sensitivity statistics, both invariant and not, in cases in which it would otherwise be impossible to obtain convergence. However, one fully exploits the increased numerical accuracy if the global sensitivity statistic is monotonic transformation invariant. Conversely, estimators of measures that do not have this invariance property might lead to misleading deductions.

Measures of sensitivity and uncertainty have become an integral part of risk analysis. Many such measures have a conditional probabilistic structure, for which a straightforward Monte Carlo estimation procedure has a double-loop form. Recently, a more efficient single-loop procedure has been introduced, and consistency of this procedure has been demonstrated separately for particular measures, such as those based on variance, density, and information value. In this work, we give a unified proof of single-loop consistency that applies to any measure satisfying a common rationale. This proof is not only more general but invokes less restrictive assumptions than heretofore in the literature, allowing for the presence of correlations among model inputs and of categorical variables. We examine numerical convergence of such an estimator under a variety of sensitivity measures. We also examine its application to a published medical case study.